What are the safest and easiest extra-cardiac biopsy sites for the diagnosis of wild-type transthyretin cardiac amyloidosis? Confirmation of the diagnosis by biopsy of the pectoralis muscle during pacemaker insertion

Background: The use of nonbiopsy diagnosis of wild-type transthyretin cardiac amyloidosis (ATTRwt-CA) has grown worldwide. However, in specific scenarios, a biopsy of the myocardium or involved tissues must be conducted to confirm deposition of amyloid transthyretin (ATTR). Case report: An 83-year-old woman was diagnosed with decompensated heart failure, sick sinus syndrome (SSS), and an atrial septal defect (ASD). Cardiac amyloidosis was also suspected. Technetium-99m-labelled pyrophosphate (Tc-99m-PYP) scintigraphy revealed that tracer uptake was greater in the right ventricular (RV) myocardium than in the left. Tc-99m-PYP uptake in many of the skeletal muscles, including the pectoralis major, was also observed. Permanent pacemaker implantation for SSS was performed without complications. A sample of the pectoralis major was resected while forming the pocket for the pacemaker and was used to confirm deposition of ATTR through Congo red staining and immunohistochemical analysis. No mutations were detected in the transthyretin gene. Therefore, the patient was diagnosed with ATTRwt-CA and treatment with tafamidis was initiated. Conclusion: Histological confirmation of amyloid type is generally needed when a patient presents with suspected immunoglobulin light chain cardiac amyloidosis, visual grade 1 myocardial Tc-99m-bone-avid isotope uptake, visual grade 2–3 uptake with plasma cell dyscrasia, or suspected infrequent types of amyloidosis. In our patient, a rare Tc-99m-PYP uptake pattern due to RV overload from ASD was observed, and a biopsy was required to confirm amyloid type. A biopsy of the pectoralis major may be performed when a permanent pacemaker is implanted, and it can be useful for confirming deposition of ATTR.